The present disclosure relates generally to an epicyclic gearbox assembly and, more specifically, to a preloaded hydrodynamic journal bearing guide for a roller bearing cage that increases the stability of the roller bearing cage while in operation.
At least some known gas turbine engines, such as turbofan engines, include a fan, a core engine, and a power turbine. The core engine includes at least one compressor, a combustor, and a high-pressure turbine coupled together in a serial flow relationship. More specifically, the compressor and high-pressure turbine are coupled through a first drive shaft to form a high-pressure rotor assembly. Air entering the core engine is mixed with fuel and ignited to form a high energy gas stream. The high energy gas stream flows through the high-pressure turbine to rotatably drive the high-pressure turbine such that the shaft rotatably drives the compressor. The gas stream expands as it flows through a power or low-pressure turbine positioned aft of the high-pressure turbine. The low-pressure turbine includes a rotor assembly having a fan coupled to a second drive shaft. The low-pressure turbine rotatably drives the fan through the second drive shaft.
The drive shafts in the turbine engine are typically supported by one or more bearings, and at least some known turbofans include a speed-reducing gearbox coupled along the drive shaft between the low-pressure turbine and the fan. The gearbox facilitates decoupling the fan tip speed from the speed of the low-pressure turbine. For example, at least some known gearboxes include a sun gear engaged with and rotatably mounted radially inward relative to a plurality of planetary gears. The planetary gears each include a roller bearing cage having a plurality of roller elements therein. In operation, the planetary gears sometimes rotate circumferentially about the sun gear, and also rotate about an axis. As the rotational speed of the planetary gears increases, rotation of the planetary gears about the sun gear induces a centrifugal field on the roller bearing cages of the planetary gears. This can result in excess loading and destabilization of the roller bearing cages from their original centered position and a reduction in the service life of the bearings. Also, dynamic unbalanced loads can arise in operation due to material and manufacturing inhomogeneities of the cage. At some operating speeds, the roller bearing cages of the planetary gears can enter an unstable operation condition due to the dynamic unbalanced loads that leads to chaotic orbiting around the bearing axis. This can result in reducing the service life of the planetary gears.